The present invention relates to an optical memory device and, more particularly, to an optical memory device which is capable of detecting an erroneous writing of data.
In an optical memory device, rows of pits are formed on the surface of an optical disc in accordance with recording data so as to write data. Pits are formed by evaporating the photosensitive material on the surface of the optical disc with a laser beam. Therefore, if the pits are not formed correctly or if there is a scratch or the like on the surface of the optical disc, data is erroneously written. In order to prevent this, a judgment must be made as to whether or not data has been correctly written. Detection of erroneous writing is conventionally performed utilizing light from the laser beam reflected off the optical disc. When a pit is formed, the reflectivity of the surface of the optical disc is varied to cause a level difference between the reflected light before and after pit formation, thus enabling detection of the presence or absence of the pit. If the level of the reflected light after the pit is formed is not lower than that of the reflected light before the pit is formed, even if a pit has been formed at the corresponding portion, correct formation of the pit has not occurred. Furthermore, if the level of the reflected light after the pit is formed is lower than that of the reflected light before the pit is formed, even if a pit has not actually been formed, it is probable that there is a scratch or the like on the optical disc. The reliability of the recording data is judged in this manner.
However, since it takes a considerable period of time after a writing laser beam is radiated before a pit is actually formed, the beam overlaps only half the pit. The level difference is not enough to detect erroneous writing. If the data writing speed is high, the overlapped portion becomes small and judgment reliability is degraded even further.